1. FIELD OF THE INVENTION
The present invention relates to pilot-controlled valves intended for use with hydraulic circuits, specifically valves of the type generally used in a braking valve, balancing valve, speed limiter or safety valve.
These valves are designed so as to prevent an abrupt and dangerous drop of the load raised, even in the event of rupture of a pipeline.
In general terms, such a valve is used to maintain a sufficient pressure in a hydraulic motor or in a hydraulic jack to allow the controlled displacement of a torque or of a drive load. As a non-limiting example, it will be noted that such a braking valve can be used on a vehicle with an open-circuit hydrostatic transmission when it descends a slope or when it carries out a braking operation, as well as on a hydraulic crane when it deposits a load on the ground.
2. DESCRIPTION OF THE PRIOR ART
The use of a braking valve of this type for controlling the hydraulic members supporting heavy loads is well known. For example, U.S. Pat. No. 4,470,339, incorporated herein by reference, shows such valve inserted in a high-pressure circuit to vary the shut-off cross-section and is controlled by means of a pilot-control pressure. However, this pilot-control pressure can be made to vary during operation in a way which is not always under control and can result in the occurrence of jerky movements in the receiving devices.
If, for example, such a valve is mounted on the control of two chambers of a hydraulic crane jack, the recording of the pressures at the start of the movement shows that the pressure decreases in the chamber of the jack which retains the load and increases in the opposite chamber. These pressure variations correspond to the phase of acceleration of the movable mass connected to the jack. When the desired speed is reached, the pressure increases in the loaded section of the jack and diminishes in the opposite section, as shown in the prior art FIG. 1. In this figure, the curve points P.sub.1, P'.sub.1, and P".sub.1 represent the pressure changes in that chamber of the jack which retains the load. In contrast to this, the points P.sub.2, P'.sub.2, and P".sub.2 represent the pressure changes in the opposite chamber of the jack. The time t is plotted on the abscissa.
This physical phenomenon gives rise to problems in the adjustment of the braking valve which, because of its design, is sensitive to pressure variations in the loaded section and the opposite section.
Another physical phenomenon linked to the operation of the jack disturbs the pressure conditions in the chambers of the jack. This phenomenon is friction. Taking as an example a jack, the efficiency of which is 0.95, retaining a mass which determines a pressure of 300 bars in the loaded section, the action of friction is equivalent to a pressure of 300. (1-0.95)=15 bars. The pressure variation in the loaded section can be .+-.15 bars depending on the direction of the average speed.
In actual fact, the average speed is adjusted as a result of the flow in the opposite section. Consequently, the influence of the variations attributed to friction is manifested in pressure variations occurring, for example, in the small section. If the ratio of the sections of the jack is 1.8, the amount of pressure variations in the small section will be: .+-.15.times.1.8=.+-.27 bars.
Finally, if the braking valve described in U.S. Pat. No. 4,470,339 is considered, the ratio between the section S, sensitive to the pilot-control pressure and the section s, sensitive to the pressure to be braked, is: EQU S/s=15
Under these conditions, it will be seen that a variation of 27 bars in the pilot-control pressure is equivalent to a variation of 27.times.15=405 bars in the pressure to be braked.
The results of these two phenomena being superimposed on one another are illustrated in FIG. 2 and schematically shown in FIG. 3. Assuming a loaded jack 1, of which the movement according to the drive load (the direction of contraction indicated by the arrow 2) is controlled by a safety valve 3 calibrated to a pressure P.sub.o and the pilot-control ratio of which is N, the opening condition of the valve is: EQU P.sub.o.sup.= P.sub.1 +P.sub.2 .multidot.N
where:
P.sub.o is the calibration threshold of the valve 3; PA1 P.sub.1 is the pressure at the large section in the jack 1; and PA1 P.sub.2 is the pressure at the small section, used to pilot-control the safety valve 3.
Without the phase of setting the jack 1 in motion, P.sub.1 drops in order to obtain an acceleration of the mass attached to the jack; let P.sub.m be the value of the pressure corresponding to the acceleration force. A corresponding pressure P.sub.f can be defined for the frictional force. During the phase of setting in motion, a pressure: EQU P'.sub.1 =P.sub.1 -(P.sub.m +P.sub.f)
will prevail in the loaded section of the jack.
To ensure that the safety valve is opened, it is necessary for P.sub.2 to increase by the value: ##EQU1##
When the speed is reached, the acceleration pressure is zero. The pressure in the jack then assumes the value P".sub.1, so that P".sub.1 =P.sub.1 -P.sub.f. Likewise, P.sub.2 becomes P".sub.2.
Thus, the pressure spectrum as a function of time is illustrated in FIG. 1, and the variations in the balanced state, when the safety valve is pilot-controlled by the pressure P.sub.2 in the opposite section, are shown in FIG. 2. However, this solution is of interest because it makes it possible to detect the runaway of the load (in the event of overspeed the pressure in the opposite section is cancelled). It also avoids the need to retransmit electrical or hydraulic commands to the safety valve and, therefore, to the jack. This latter point is especially useful for machines equipped with manual or muscular-control distributors and having as their only command system displacements of mechanical components, such as levers and connecting rods, between the operator and the group of distributors.